Another cause may be taking your insulin in the same spot. Have you tried different areas, tummy, leg, arms? It's important to rotate areas because lumps of insulin can gather under the skin and new insulin injected there will not work as well.
Just a tip, when I'm high i inject into my muscle, top upper arm for quick response. It doesn't hurt and if it doesn't bleed you're good to go. If it bleeds after keep testing to make sure it works - it always worked for me blood or not but that's what i was told.
Good luck